The present invention relates to a method and an apparatus for calibration of well logging systems; more particularly, it relates to a method and an apparatus for calibration and verification of the porosity response of neutron logging instruments.
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data, such as porosity, lithology and water saturation, for use by geologists and petroleum engineers in making determinations pertinent to hydrocarbon exploration. One such system is commonly referred to as a neutron log.
The neutron log is primarily used for the estimation of formation porosities. A neutron logging instrument includes a neutron source and a detector capable of detecting neutron radiation and/or gamma radiation resulting from neutron capture. The basic principle of the neutron log is related to the slowing down of neutrons by the nuclei of the substances encoyntered in the formations. The radiation flux density in the vacinity of the detector is influenced by the composite hydrogen index of the media between the source and the detector. If the rock matrix in the formations contains liquid hydrocarbons and/or water, the neutron log provides porosity with acceptable porosity.
As in the case of most well logging systems, the neutron logging instrument requires calibration and verification by relating the measurement to a known standard. One common method for calibration and verification is accomplished by the use of calibration "test pits" which include a plurality of individual boreholes in a plurality of formation materials of known porosities into which the neutron logging instrument is disposed. While test pits provide accuracy for calibration and verification they have several disadvantages, including high expense of installation and a fixed location. In addition, the requirements of the test pits does not allow installation at each well site. Typical portable calibrators have suffered from numerous short comings, including a lack of sufficient accuracy; a limited number of porosity measurements, typically only one for each calibrator; and adverse affect by environmental factors.
These and other disadvantages are overcome with the present invention by providing a method and an apparatus for calibration and verification of the response of neutron logging instruments by producing selectable and precisely repeatable environments that represent an extensive range of formation porosities.